1. Field of the Invention
The invention relates to liquid crystal display devices and fabrication methods thereof, and more particularly to optically compensated bend mode liquid crystal display (OCB-LCD) devices and fabrication methods thereof.
2. Description of the Related Art
Liquid crystal display (LCD) devices have several advantages over other display technologies, such as a smaller volume, a lighter weight, and lower power consumption. As such, LCD devices are being applied in a variety of electronic and communication devices including notebook computers, personal digital assistants (PDA), mobile phones and others. Given the trends, technological development of LCD devices are now focusing on lighter and thinner profiles with increased portability.
However, for conventional LCD devices, due to a narrow viewing angle applications have been limited. To improve the viewing angle of LCD devices, multi-domain vertical alignment (MVA) mode LCD devices comprising of bumps or protrusions on substrate for creating different orientations of liquid crystal molecules have been introduced. Nonetheless, different liquid crystal orientations can cause electric field changes in a single liquid crystal cell, changes in liquid crystal alignment and changes in liquid crystal relaxation. In addition, forming bumps or protrusions on substrate requires a complex lithographic process utilizing a half-tone mask.
Another conventional method for improving the viewing angle of LCD devices is provided by changing orientations of liquid crystal molecules to achieve self-compensated viewing angles. This method improves response speed and widens viewing angles of LCD devices.
U.S. Pat. No. 6,853,435, the entirety of which is hereby incorporated by reference, discloses an optically compensated bend mode liquid crystal display (OCB-LCD) device with fast response speed and wide viewing angles. Since the display mode requires transition from a splay mode to a bend mode by switching, the transition time takes from several seconds to several minutes. Thus, in order to reduce the transition time, protrusion structures are disposed on the lower substrate, resulting in changes of the electric flux lines and thereby dramatically reducing transition time.
FIG. 1 is a cross section of a conventional OCB-LCD device. Referring to FIG. 1, a conventional OCB-LCD device 100 includes a first substrate 108 and a second substrate 101 opposing to each other and with a spacer 105 interposed therebetween. A pixel electrode 107 is disposed on the first substrate 108, and a lower alignment layer is disposed on the pixel electrode 107. A common electrode 102 is disposed on the second substrate 101. An upper alignment layer 103 is disposed on the common electrode 102. A liquid crystal fills the gap between the first substrate 108 and the second substrate 101. Protrusion structures 110 are formed on the lower substrate of the conventional OCB-LCD device 100 resulting in changes of the electric flux lines such that the transition time is dramatically decreased.
U.S. Pat. No. 6,535,259, the entirety of which is hereby incorporated by reference, discloses another OCB-LCD device with pixel fringe regions between two adjacent pixels. Liquid crystal molecules within pixel fringe regions are dominated by two fringe fields, resulting in unstable liquid crystal distribution and increased transition. Furthermore, by using additional protrusion structures on the lower substrate and the abovementioned boundary conditions, inclinations of the liquid crystal molecules are stabilized.
FIG. 2 is a cross section of another conventional OCB-LCD device. Referring to FIG. 2, a conventional OCB-LCD device includes a first substrate 220 and a second substrate 210 opposing to each other and with a specific gap therebetween. The first substrate is an active substrate with date lines 221 and active devices 222 such as thin film transistors (TFTs) thereon. A passivation layer 223 is formed overlying the first substrate 220 covering the date lines 221 and active devices 222. Protrusion structures 226 are formed on the data lines 221. A pixel electrode 225 is disposed on the first substrate 220 and electrically connected to the active devices 222. A first alignment layer 241 is disposed on the first substrate. After rubbed along the rubbing direction R, the surface of the first alignment layer 241 creates an anchor force against and inclining the LC molecules.
The second substrate 210 is a color filter (CF) substrate with color filters 203 corresponding to each sub-pixels and a black matrix layer 202 among the color filters 203. A common electrode 204 is disposed on the color filters 203 and black matrix layer 202. A second alignment layer 242 is formed on the common electrode 204 of the second substrate 210. After rubbed along the rubbing direction R, the surface of the second alignment layer 242 creates an anchor force against and inclining the LC molecules. A liquid crystal layer 230 fills gap between the first substrate 220 and the first substrate 210. Protrusion structures 226 of the TFTs 222 and data lines 221 on the lower substrate of the conventional OCB-LCD device 100 can result in changes of the electric flux lines such that the transition time is dramatically reduced.
Moreover, another method to improve transmittance of the conventional OCB-LCD devices is disclosed to change driving pixel electrodes by observing mechanisms of the splay-to bend (S-B) transition. For example, Samsung Electronics in 2006 annual conference of the society for information display (SID) discloses an OCB-LCD device with effectively 20% brightness improvement revealed from applied voltage vs. transmittance characteristics. Furthermore, Chunghwa Picture Tubes, LTD. in 2006 annual conference of the society for information display (SID) discloses that by changing rubbing direction, transmittance and brightness of the OCB-LCD devices can be improved.
U.S. Pat. No. 6,927,825, the entirety of which is hereby incorporated by reference, further discloses an OCB-LCD device. The splay-to-bend (S-B) transition can be access rated by reducing interval between pixel regions. In order to fulfill high S-B transition, fast response, and high brightness, the pre-tilt angles of liquid crystal molecules are approximately in a range of 1.2° to 3°.